Water is integral to the operation of a fuel cell system, for example in the form of the system described herein comprising a fuel cell stack based around a proton exchange membrane (PEM). Reaction of protons (hydrogen ions) conducted through the PEM from an anode flow path, with oxygen present in a cathode flow path, produces water. Excess water needs to be removed from the fuel cell stack to avoid flooding and causing a consequent deterioration in performance. An amount of water, however, needs to be present in at least the cathode flow path to maintain hydration of the PEM, so as to achieve optimum performance of the fuel cell. Managing this water, by deliberate injection and removal, can also provide a useful mechanism for removing excess heat from the fuel cell stack.
To optimize performance, water can be employed deliberately in such fuel cell systems through injection into the cathode flow path of the stack. Such water injection fuel cell systems have potential advantages of reduced size and complexity, as compared with other types of fuel cell systems employing separate cooling channels. Water may be injected directly into the cathode flow path through water distribution manifolds, as for example described in GB2409763.
For water injection systems, it is important that any water fed back into the cathode flow path is of high purity, so as to avoid contamination of the PEM and consequent degradation of stack performance. This requirement for high purity, however, means that additives to lower the freezing point of water cannot be used. For automotive applications in particular, typical requirements include starting up from below freezing, typically as low as −20° C. to replicate environments in which the fuel cell may be used in practice. Since high purity water has a freezing point of 0° C. (at 1 bar pressure), any water left in the fuel cell system will, given sufficient time, freeze after shut-down of the fuel cell.
Ice in the fuel cell system, and in particular within the cathode flow path, can prevent the stack from operating properly, or even at all. If any part of the cathode flow path is blocked with ice, air cannot be passed through the cathode and the fuel cell may not be capable of self-heating to above freezing point. Other methods of heating the whole stack will then be necessary, which will require consumption of external power before the fuel cell can begin supplying electrical power and heat by itself.
A purging operation can be used on shut-down of a fuel cell stack, such as that described in U.S. Pat. No. 6,479,177. This document discloses a fuel cell stack having water cooling passages separate from the cathode flow path. A pressurised dry nitrogen feed is used to purge water from the stack before allowing the temperature of the stack to fall below freezing. This method, however, requires a supply of pressurised nitrogen, which might not be available or even desirable in an automotive environment.